Messenger RNA (mRNA) directs protein synthesis. As a therapeutic strategy, antisense therapy strives to disrupt the synthesis of target proteins by using a sequence-specific oligonucleotide to form a stable heteroduplex with its corresponding mRNA. Such antisense oligonucleotides generally have been natural phosphodiester oligonucleotides.
As contrasted to natural phosphodiester oligonucleotides, the use of phosphorothioate, methylphosphonate, phosphotriester or phosphoramidate oligonucleotides in antisense therapy provides certain distinguishing features. Each of the phosphorothioate, methylphosphonate, phosphotriester or phosphoramidate phosphorus linkages can exist as diastereomers. Certain of these phosphorothioate, methylphosphonate, phosphotriester or phosphoramidate oligonucleotides have a greater resistance to nucleases. Some have solubilities similar to the solubility of natural phosphodiester oligonucleotides. Other have solubilities different from that of the natural phosphodiester oligonucleotides. Some are generally more chemically or thermodynamically stable than the natural phosphodiester oligonucleotides. At least the phosphorothioates have oligonucleotide-RNA heteroduplexes that can serve as substrates for endogenous RNase H.
The phosphorothioate oligonucleotides, like the natural phosphodiester oligonucleotides, are soluble in aqueous media. In contrast, methylphosphonate, phosphotriester, and phosphoramidate oligonucleotides, which lack a charge on the phosphorus group, can penetrate cell membranes to a greater extent and, thus, facilitate cellular uptake. The internucleotide linkage in methylphosphonate oligonucleotides is more base-labile than that of the natural phosphodiester internucleotide linkage, while the internucleotide linkage of the phosphorothioate oligonucleotides is more stable than the natural phosphodiester oligonucleotide linkage.
The resistance of phosphorothioate oligonucleotides to nucleases has been demonstrated by their long half-life in the presence of various nucleases relative to natural phosphodiester oligonucleotides. This resistance to nucleolytic degradation in vitro also applies to in vivo degradation by endogenous nucleases. This in vivo stability has been attributed to the inability of 3'-5' plasma exonucleases to degrade such oligonucleotides. Phosphotriester and methylphosphonate oligonucleotides also are resistant to nuclease degradation, while phosphoramidate oligonucleotides show some sequence dependency.
Since they exist as diastereomers, phosphorothioate, methylphosphonate, phosphotriester or phosphoramidate oligonucleotides synthesized using known, automated techniques result in racemic mixtures of Rp and Sp diastereomers at the individual phosphorothioate, methylphosphonate, phosphotriester or phosphoramidate linkages. Thus, a 15-mer oligonucleotide containing 14 asymmetric linkages has 2.sup.14, i.e. 16,384, possible stereoisomers. Accordingly, it is possible that only a small percentage of the oligonucleotides in a racemic mixture will hybridize to a target mRNA or DNA with sufficient affinity to prove useful in antisense or probe technology.
Miller, P. S., McParland, K. B., Jayaraman, K., and Ts'o, P.O.P (1981), Biochemistry, 20:1874, found that small di-, tri- and tetramethylphosphonate and phosphotriester oligonucleotides hybridize to unmodified strands with greater affinity than natural phosphodiester oligonucleotides. Similar increased hybridization was noted for small phosphotriester and phosphoramidate oligonucleotides; Koole, L. H., van Genderen, M. H. P., Reiners, R. G., and Buck, H. M. (1987), Proc. K. Ned. Adad. Wet., 90:41; Letsinger, R. L., Bach, S. A., and Eadie, J. S. (1986), Nucleic Acids Res., 14:3487; and Jager, A., Levy, M. J., and Hecht, S. M. (1988), Biochemistry, 27:7237. The effects of the racemic diastereomers on hybridization becomes even more complex as chain length increases.
Bryant, F. R. and Benkovic, S. J. (1979), Biochemistry, 18:2825 studied the effects of diesterase on the diastereomers of ATP. Published patent application PCT/US88/03634 discloses dimers and trimers of 2'-5'- linked diastereomeric adenosine units. Niewiarowski, W., Lesnikowski, Z. J., Wilk, A., Guga, P., Okruszek, A., Uznanski, B., and Stec, W. (1987), Acta Biochimica Polonia, 34:217, synthesized diastereomeric dimers of thymidine, as did Fujii, M., Ozaki, K., Sekine, M., and Hata, T. (1987), Tetrahedron, 43:3395.
Stec, W. J., Zon, G., and Uznanski, B. (1985), J. Chromatography, 326:263, have reported the synthesis of certain racemic mixtures of phosphorothioate or methyphosphonate oligonucleotides. However, they were only able to resolve the diastereomers of certain small oligomers having one or two diastereomeric phosphorus linkages.
In a preliminary report, J. W. Stec, Oligonucleotides as antisense inhibitors of gene expression: Therapeutic implications, meeting abstracts, Jun. 18-21, 1989, noted that a non-sequence-specific thymidine homopolymer octomer--i.e. a (dT).sub.8 -mer, having "all-except-one" Rp configuration methylphosphonate linkages--formed a thermodynamically more stable hybrid with a 15-mer deoxyadenosine homopolymer--i.e. a d(A).sub.15 -mer--than did a similar thymidine homopolymer having "all-except-one" Sp configuration methylphosphonate linkages. The hybrid between the "all-except-one" Rp (dT).sub.8 -mer and the d(A).sub.15 -mer had a Tm of 38.degree. C. while the Tm of the "all-except-one" Sp (dT).sub.8 -mer and the d(A).sub.15 -mer was &lt;0.degree. C. The hybrid between a (dT).sub.8 -mer having natural phosphodiester linkages, i.e. octathymidylic acid, and the d(A).sub.15 -mer was reported to have a Tm of 14.degree. C. The "all-except-one" thymidine homopolymer octamers were formed from two thymidine methylphosphonate tetrameric diastereomers linked by a natural phosphodiester linkage.
To date, it has not been possible to chemically synthesize an oligonucleotide having more than two adjacent, chirally pure phosphorous linkages. Indeed, even in homopolymers it has been possible to produce only three such adjacent chiral linkages. For an oligonucleotide to be useful as an antisense compound, many nucleotides must be present. While not wishing to be bound by any particular theory, it is presently believed that generally at least about 10 or more nucleotides are necessary for an oligonucleotide to be of optimal use as an antisense compound. Because it has not been possible to resolve more than two or three adjacent phosphorus linkages, the effects of induced chirality in the phosphorus linkages of chemically synthesized antisense oligonucleotides has not been well assessed heretofore.
Except as noted above, the sequence-specific phosphorothioate, methylphosphonate, phosphotriester or phosphoramidate oligonucleotides obtained utilizing known automated synthetic techniques have been racemic mixtures. Indeed, it was recently stated in a review article that: "It is not yet possible to synthesize by chemical means diastereomerically pure chains of the length necessary for antisense inhibition," see J. Goodchild (1990) Bioconjugate Chemistry, 1:165.
The use of enzymatic methods to synthesize oligonucleotides having chiral phosphorous linkages has also been investigated. Burgers, P. M. J. and Eckstein, F. (1979), J. Biological Chemistry, 254:6889; and Gupta, A., DeBrosse, C., and Benkovic, S. J. (1982) J. Bio. Chem., 256:7689 enzymatically synthesized diastereomeric polydeoxyadenylic acid having phosphorothioate linkages. Brody, R. S. and Frey, P. S. (1981), Biochemistry, 20:1245; Eckstein, F. and Jovin, T. M. (1983), Biochemistry, 2:4546; Brody, R. S., Adler, S., Modrich, P., Stec, W. J., Leznikowski, Z. J., and Frey, P. A. (1982) Biochemistry, 21:2570-2572; and Romaniuk, P. J. and Eckstein, F. (1982) J. Biol. Chem., 257:7684-7688 all enzymatically synthesized poly TpA and poly ApT phosphorothioates while Burgers, P. M. J. and Eckstein, F. (1978) Proc. Natl. Acad. Sci. USA, 75:4798-4800 enzymatically synthesized poly UpA phosphorothioates. Cruse, W. B. T., Salisbury, T., Brown, T., Cosstick, R. Eckstein, F., and Kennard, O. (1986), J. Mol. Biol., 192:891, linked three diastereomeric Rp GpC phosphorothioate dimers via natural phosphodiester bonds into a hexamer. Most recently Ueda, T., Tohda, H., Chikazuni, N., Eckstein, R., and Watanabe, K. (1991) Nucleic Acids Research, 19:547, enzymatically synthesized RNA's having from several hundred to ten thousand nucleotides incorporating Rp diastereomeric phosphorothioate linkages. Enzymatic synthesis, however, depends on the availability of suitable polymerases that may or may not be available, especially for modified nucleoside precursors.
Thus, while phosphorothioate, alkylphosphonate, phosphoamidate, and phosphotriester oligonucleotides have useful characteristics, little is known concerning the effects of differing chirality at the phosphorus linkages. It would therefore be of great advantage to provide oligonucleotides having phosphorous linkages of controlled stereochemistry.